The present invention relates to threaded fasteners and to sheet metal screws in particular. The invention is related to co-pending applications Ser. No. 426,459 filed Sept. 29, 1982 and Ser. No. 316,260 filed Oct. 29, 1981.
The present invention relates to the idea of increasing stripping torque. The use of thinner sheet metal in appliances and in automobiles has made screws more prone to stripping. The use of automatic driving equipment having settable clutching mechanisms makes it important to maximize the difference between the torque required to drive a screw and the torque at which the screw strips out the hole in the sheet metal. This is generally achieved by increasing the strip torque rather than by lowering drive torque.
The concept of increased strip torque should not be confused with the concept of increased removal torque or locking. While the two concepts may be related in that efforts to improve one may result in an improvement of the other, the concepts are sometimes diametrically opposed in that improvements to one may have to be made at a sacrifice to the other.
Frequently, increased strip torque and removal torque are achieved by forming sharp projections of one sort or another on the under surface of a screw. This has at least two drawbacks. First, damage to the surface of a workpiece may result, and secondly, such projections are difficult to manufacture because dies used wear quickly.
Accordingly, it is an object of the present invention to provide a screw which increases the strip torque of a sheet metal screw.
Another object is to provide a screw having greater strip torque and yet does minimal damage to the surface of a workpiece.
Another object is to provide a screw having increased strip torque which is simple to manufacture.
Another object is to provide a screw having increased strip torque and increased removal torque.
These and other objects and advantages are obtained with a screw having a head with an integral undulating flange. The flange is sloped so that variations in the radial configuration of the flange necessarily result in axial variations. Undulations caused by the radial and axial variations interfere with the surface of the workpiece and increase the amount of torque required to strip the screw. The inference is both axial and radial and yet causes a minimum of damage to the workpiece because the undulations are not abrupt.
The invention will be better understood upon a reading of the following specification, read in conjunction with the accompanying drawings.